As applications for pouch-type lithium ion secondary batteries have expanded, an energy density and storage amount in the unit cell have increased. In particular, to meet the recent demands for an increase in driving distance, high energy technologies have been applied to a lithium ion secondary battery for a vehicle.
However, as the energy of a lithium ion battery increases, safety of the lithium ion battery can be compromised. That is, as the amount of energy which may be stored is increased, the number of risk factors, such as ignition and explosion, also increases. In particular, in the case of a pouch cell which does not have cell level safety devices (e.g., a current interrupt device (CID), a positive temperature coefficient thermistor (PTC), a fuse, and the like), but yet has an exterior material with weak physical strength, safety issues corresponding to the increase in energy have occurred.